Prosthetic ankle devices are frequently used as replacement after the loss of lower limb following amputation. Prostheses can fulfill both the aesthetic and the functional role of the lower leg, such as running, sports, or other exercise; climbing or descending stairs; ascending or descending slopes; level walking; other movement; and restoration of the appearance of the missing limb.
A sound human ankle helps a person walk during gait. The phase of gait where the foot touches the ground is known as the stance phase of gait. In the initial part of stance phase (from heel-strike to mid-stance), the ankle stores elastic energy in the elongation of its tendons. In late-stance (from mid-stance to toe-off), the energy stored in the tendons is returned, along with the addition of an active muscle-powered component. This energy propels a person forward while walking. Such behavior cannot be replicated by passive or quasi-passive prosthesis that require the user to supply this missing energy, such as by pushing forward the user's body center of mass on toe-off; through an increased torque in the remaining lower-limb joints (mainly the hip); or by altering the symmetry of the gait between the two limbs.
Some prostheses are fully powered. Powered ankle prostheses have the potential to provide substantial benefits for amputees and provide further opportunities for clinical research. However, powered ankle prostheses known in the art have drawbacks in technology and implementation. Achieving one or more of the design goals of appropriate battery duration, structural strength, high range of motion and lightness are difficult to meet while also enclosing the design of the ankle prosthesis into an anatomical shape, such as a shape that would fit within the user's shoe. Moreover, interchangeability and modularity (such as interfacing with the stump's socket, or pylon or torsion-elements) can be a problem due to the prosthesis dimensions and built-height, especially for transtibial amputees.